1. Field of the Invention
The present invention relates to a liquid crystal display, particularly to a bend alignment mode liquid crystal display.
2. Description of the Related Art
Liquid crystal displays (LCDs) are advantageous in smaller thicknesses, lighter weights, lower power consumptions, etc. as compared with cathode ray tubes (CRTs). The liquid crystal displays have a liquid crystal cell and a pair of polarizing plates placed at both sides of the cell. The liquid crystal cell contains liquid crystal molecules, two substrates for enclosing the molecules, and an electrode layer for applying voltage to the molecules. An alignment layer is generally formed on the substrates to align the enclosed liquid crystal molecules. An optical compensatory sheet (a retardation sheet) is often formed between the liquid crystal cell and the polarizing plates to remove undesired coloration of an image in the liquid crystal cell. The stack of the polarizing plates (polarizing films) and the optical compensatory sheet acts as an ellipsoidal polarizing plate. The optical compensatory sheet can have a function of enlarging a viewing angle of the liquid crystal cell. Stretched birefringent films have been used as the optical compensatory sheet.
Also optical compensatory sheets having an optically anisotropic layer containing a discotic compound, which can be used instead of the stretched birefringent films, have been proposed in JP-A-6-214116, U.S. Pat. Nos. 5,583,679 and 5,646,703, German Patent No. 3,911,620 A1, etc. The optically anisotropic layer is formed by aligning the discotic compound and by fixing the alignment state. The discotic compound generally has a large birefringence. Further, the discotic compound can be in various alignment states. Thus, the discotic compound can be used for producing an optical compensatory sheet with nonconventional optical properties, which the stretched birefringent films cannot have.
For example, proposed in U.S. Pat. Nos. 4,583,825 and 5,410,422, etc. are liquid crystal displays using a bend alignment mode liquid crystal cell containing rodlike liquid crystal molecules. In the bend alignment mode liquid crystal cell, the rodlike liquid crystal molecules in the upper portion and the molecules in the lower portion are aligned in substantially opposite directions (symmetrically), whereby the liquid crystal cell has a self-optical compensatory function. Thus, the liquid crystal mode is referred to also as an OCB (Optically Compensatory Bend) liquid crystal mode. The bend alignment mode liquid crystal displays are advantageous in high response speeds.
The bend alignment mode is characterized in wider viewing angle and higher response speed as compared with common liquid crystal modes such as TN and STN modes. However, in comparison to the CRTs, even the bend alignment mode displays have to be further improved. It seems possible that the bend alignment mode liquid crystal displays can be further Improved by using an optical compensatory sheet as the common liquid crystal mode displays. However, the conventional optical compensatory sheets of the stretched birefringent films show insufficient optical compensatory function in the bend alignment mode liquid crystal displays. Use of the optical compensatory sheets having a transparent support and an optically anisotropic layer containing a discotic compound instead of the stretched birefringent films has been proposed as described above. Also bend alignment mode Liquid crystal displays using an optical compensatory sheet containing a discotic compound have been proposed in JP-A-9-197397, WO 96/37804, etc. By using the optical compensatory sheet containing a discotic compound, viewing angle of the bend alignment mode liquid crystal displays is remarkably improved.
It has been pointed out in JP-A-11-316378, etc. that, when the optical compensatory sheet containing a discotic compound is used in the bend alignment mode liquid crystal displays, a particular wavelength light leaks to cause undesired coloration of an image. It is reported that the coloration is caused because transmittance of an ellipsoidal polarizing plate (a stack of polarizing films and an optical compensatory sheet) depends on wavelength. Further, it is reported that a maximum optical compensatory effect for a bend alignment mode liquid crystal cell can be achieved by arranging an optically anisotropic layer and polarizing films such that an angle of an average orthographic projection of a discotic plane normal line of a discotic compound to an optically anisotropic layer against an in-plane transmission axis of polarizing films is substantially 45°.
Various proposals have been made on reduction of undesired coloration and prevention of tone reversal in the bend alignment mode liquid crystal displays using the optical compensatory sheet containing a discotic compound in Japanese Patent No. 3056997, JP-A-2002-40429, etc.